Squaring circuits produce an output voltage that is proportional to the square of an input voltage and can be useful to detect the root mean squared (RMS) power contained in the envelope of a modulated signal. Squaring circuits may be used in the automatic gain control (AGC) implementation for a TV tuner or any receiver having a variable gain amplifier. Additionally, squaring circuits may be used in the AGC loop to detect and control the output power from a transmitter. Conventional circuits for power detectors rely on bipolar transistors or Schottky diodes for RMS power detection. These conventional circuits cannot be easily integrated with commonly used CMOS technology in the form of a monolithic integrated circuit and thus MOS squaring circuits are desirable.
One true squaring circuit that is well know includes a pair of dual-ended difference amplifiers that each receives an input signal, a pair of dual-to-single-ended converters that receive the respective dual-ended output of the difference amplifiers, and a summing network for summing the squares of the outputs of the dual-to-single-ended converters to suppress signal dependent error terms from being output. While this circuit is useful for squaring a balanced input, it is susceptible to fabrication process dependent variations that affect the accuracy of the output and thus cannot be used in applications requiring less than 1 dB accuracy.
A squaring detector based on the square-law property of a pair of MOS transistors is also known. In this detector, the gate of one of the MOS transistors is excited by the input signal and the gate of the other MOS transistor is kept biased at a constant voltage. The difference in the drain currents of this MOS pair is then used to detect the power level of the input signal. This approach is also susceptible to fabrication process dependent variations that affect the accuracy of the output and thus it cannot be used in applications requiring less than IdB accuracy.
Accordingly, an improved RMS power detection circuit is desirable.